A modern detergent is a complicated mixture of a great number of different compounds. The major part of these can be assigned to the groups (i) surfactants, i.e. surface active agents, and (ii) builders, which to a large extent consist of chelating agents. The builders alone have no or little washing power. However, they strongly increase the efficiency of the surface active agents.
The knowledge of the interraction between the surface active agents and the chelating agents has been empirical. The present invention is concerned with an amplification of the conventional washing theory and considers the interaction between builders and surface active agents. It also presents teachings which can be used to provide washing, dishwashing and cleaning agents with new and from many points of view surprising properties without having to use substances which can be considered suspicious from the views of corrosion, toxicity and pollution.
An ideal detergent should have the following objectives:
1. It must be nontoxic both to the production workers and to the consumer.
2. It should have good detergency.
3. It should, independent of the properties of the diluting agents, give consistently good results.
4. It should not, during the cleansing procedure or when the cleaning solution is diluted, give adverse subsidiary effects (e.g. precipitation of insoluble compounds on or in the textile fibres).
5. It should be biologically degradable.
6. It must not damage the materials, machines or instruments used with the detergent.
7. The degradation products produced must not be toxic or otherwise disturb the ecological balance.
8. The supply of raw materials at acceptable prices should be good.
The largest group of cleaning agents in accordance with the present invention is the clothes washing agents. The situation today in relation to the above mentioned objectives follows: dominant part of the washing agents can be divided into the following main classes with reference to the builders:
I. phosphate based PA1 II. NTA (nitrilotriacetic acid) - based PA1 III. based on substitutes for phosphate other than NTA PA1 A. based on soaps PA1 B. based on synthetic anionic surface active agents PA1 C. based on nonionic surface active agents PA1 D. based on mixtures of two or three of the above groups A, B and C
These above mentioned three classes can all, with reference to the content of surface active agents, be divided into the following subdivisions:
As to the subdivisions, it can be pointed out that Group A in particular does not fulfill the points 2, 3 and 4 of the afore-listed objectives. Soap is a very effective complexing agent for the hard water components, i.e., the divalent calcium and magnesium ions usually referred to herein as "Ca.sup.2+ " and "Mg.sup.2+ ". As a consequence during washing in hard water, insoluble lime soaps are formed which remain in the washed textile material and render it rough and give it a hard or coarse feel after repeated washing. Even if complexing agents with larger stability constants than those of the soap, which is not practically applicable today, were admixed with soap in the detergent, lime soaps would be formed.
The surface active agents of group B are also somewhat sensitive to hardness formers, however to a considerably lesser degree than are the soaps. Most of the surface active agents for sale today are biologically degradable. The rate of degradation however varies within very wide limits.
The surface active agents of Group C contain some agents which are little or not at all biologically degradable. The group also contains others which are entirely biologically degradable, e.g. alkyl ethylene oxide condensates.
Group D has primarily the same disadvantages and advantages as the compounds of the mixture individually, except for one respect, namely the washing effect. An increase of the washing effect can be obtained through mixing different types of surface active agents.
The following applies with reference to the head Classes I, II and III:
I. Phosphate has a nourishing (fertilizing) effect on algae growth. Uncontrolled discharge of phosphate rich waste water causes the receiving body to be choked with algae. During the growth of algae, oxygen is consumed and gradually all organic life in the overfertilized receiving body dies. The phosphate is usually present in the detergents in the form of tripolyphosphate. This provides within certain limits soluble compounds with the hard water components, but when diluted during the rinsing the lower concentration limit for forming soluble compounds is passed, and precipitation of insoluble Ca- and/or Mg-phosphates occur in variable amounts in the washed textile, which thereby become rough and feel "hard".
II. NTA is used as a substitute for phosphate in detergents, among other things, in order to reduce the outlet of phosphates in the waste water. The question of the eventual negative biological effects of NTA is not finally established but is still being discussed. NTA is such a strong chelating agent for heavy metals that certain measures have to be taken (described in the British Pat. No. 1.162.090) to protect elements, bearings and similar details of Cu- or Zn-alloys from corrosion. NTA-detergents usually also contain a certain amount of phosphates which means that they still to a certain degree contribute to the nourishment of the bodies of water.
III. The only commercially used substitute for phosphates today which falls under this group is sodium citrate. This compound is biologically degradable. It does not damage washing machines and the like and it is nontoxic. Detergents based on sodium citrate give satisfactory washing results in normal waters. Unfortunately, the citrate based detergents for sale up to now have a big disadvantage.
When washing in medium hard and hard waters significant amounts of insoluble salts of hard water components are precipitated in the washed textiles during the rinsing. The amount of precipitate is in many cases ten times greater than when using phosphate- or NTA-based detergents. This means that the citrate based detergents can not be used advantageously except in rather soft water. Furthermore, citrate is not a very attractive raw material for detergent manufacture, as the supply of citrate is limited and consequently the prices rather high.
For an analysis of the cleaning process we can imagine surfaces (textile fibres, china, etc.) soiled with different substances. These can be soluble or insoluble in water. In this connection we do not have to worry about the water soluble substances as we are talking of wet cleaning. The water insoluble soil (dirt) can be a grease film or particles. Coarse particles do not cause any trouble either as they are weakly adherent and will be removed mechanically. Thus the cleaning process can be analysed by studying the interaction between surface active agents, grease, fibre surfaces (or other surfaces which are to be cleaned) and small particles. The present invention aims to present detergents which in their preferred forms accomplish all the above mentioned objectives.
As a rule, cleaning solutions contain relatively high proportions of surface active agents which are above the critical concentration for micelle formation, i.e., the surface active agents are present in the form of micelles.
The micelles are thought to adhere to every available surface and partly to even force their way into fibres, and between fibres and particles. The micelles will dissolve fatty soil (dirt) which in this way is loosened and emulsified.
Between all particles, emulsion drops and other free surfaces, powers of repulsion occur and also steric obstacles due to the thickness of the hydrophilic layer. In this way the loosening and separation of fatty (dirt) soil, as well as small particles from the surface which is to be cleaned, can be explained theoretically based on the so-called electrokinetic- or Z-potential between different surfaces in the cleaning solution.
The state of the formed micelle layer is, as are all physico-chemical states, not static but dynamic, so a small portion of the Ca.sup.2+ and/or the Mg.sup.2+ -ions can force their way forward to the fibre surfaces and become bound to the polar groups thereof.
The anionic surface active agents, because of their ionic character, render the important effect of imparting negative charges to the surface (substrate), while on the other hand, the non-ionic surface active agents impart a negligible addition of charge to the surface. The Ca.sup.2+ and/or Mg.sup.2+ -ions have the serious effect of neutralizing such charges. This acts to reduce the forces of repulsion between the micelles. This makes it possible for the Van der Waals forces to maintain the dirt affixed to the substrate. To prevent this, it has been necessary when using available detergents to use substances with large stability constants for Ca.sup.2+ and/or Mg.sup.2+ complexes in order to lower the proportion of free Ca.sup.2+ and/or Mg.sup.2+ -ions in the cleaning solution. In spite of that there is, as described above, precipitation on textile fibres and the like during rinsing, when the proportion (concentration) of the substance with power to chelate Ca.sup.2+ and/or Mg.sup.2+ is not sufficiently high.
During the washing process the fibres swell because of the influence of water and alkali. A portion of the fibre becomes amorphous. The amorphous substrate is connected with the surface of the fibre by narrow pores. On these surfaces negative ions (anions) are adsorbed, preferably those having a small ionic radius. There are steric obstacles to cationic--and larger anionic ions--the cationic ions because of the fact that they are surrounded by a firmly bound water layer.
The tendency of the negative ions to become adsorbed can be described in different ways. One way is for example Hofmeister's lyotropic series which is applicable to neutral solutions: EQU F.sup.-, ClO.sub.4.sup.-, SO.sub.3.sup.2-, CO.sub.3.sup.2+, PO.sub.4.sup.3-, OH.sup.-, NO.sub.3.sup.-, Br.sup.-, HS.sup.-, J.sup.-, SCN.sup.-, S.sup.2-
There is, from left to right, increasing molecular polarization, adsorption and peptization power, as well as reducing coagulating power.
In today's detergent solutions the most common negative ion is CO.sub.3.sup.2- which in many cases is a component of the detergent. Additionally it is a component of hard water. In the latter case it is present as HCO.sub.3.sup.- -ion which under the influence of alkali and/or of heat is converted into CO.sub.3.sup.2- -ion. The positive counterions of the negative ions during the washing process mostly consist of alkali metal ions such as Na.sup.+ which, due to earlier mentioned reasons, do not enter the inner structure of the fibre.
During the rinsing, the counterions are to a great extent exchanged by Ca.sup.2+ and Mg.sup.2+ coming from the hard water. When the anions due to diffusion have reached the fibre surface or parts of the fibre, where they can react with Ca.sup.2+ - and Mg.sup.2+ -ions, crystals are formed, which to a great extent stay in the fibre or between different fibres. The condition for crystal formation is of course that the negative ions are such as to give insoluble salts with Ca.sup.2+ and Mg.sup.2+ (e.g. CO.sub.3.sup.2-).
As a summing up, it can be said that the greatest drawback to washing, dishwashing and cleaning agents of today, i.e., their hard water sensitivity, can be eliminated if the amount of CO.sub.3.sup.2- and negative ions like it, which give insoluble Ca.sup.2+ and/or Mg.sup.2+ salts, is reduced to a minimum in the detergent composition. The negative ions which instead are incorporated need not be strong chelating agents of Ca.sup.2+ and Mg.sup.2+.